Yesterday I had a big surprice: I found a very red (and rare) copepode. It is Peltidium purpureum. (I did not reiforce the red colour with Photoshop ecc.!).
4 pictures in DIC. The last photo Nr. 5 is a drawing from: Conway (2012) Marine zooplankton of Southern Britain. Part 2, page 127; free download, Marine Biological Association of the United Kingdom, Plymouth.
I would like to know more about the biological function of this red colour!
I have never found this in the plankton. The "normal" habitat for this species is on seaweeds and i have found it on (and eating) the red seaweed Gracilaria sp. I think that it occurs in the plankton either by accident or as a dispersal mechanism.
In most crustacea, the colour is due to the carotenoid astaxanthin which may occur in a free form (orange) or bound to protein which may modify the colour to blue (eg lobster) or green. But I do not know if this distinctive purple is due to astaxanthin. Astaxanthin acts as an antioxidant and will protect lipids from degradation and will protect against high light.
As the organism lives on seaweeds in shallow water, the pigment may indeed be important for protection against high irradiation, but I suspect the main function may be for camouflage amongst the seaweed.
The pigment may not be astaxanthin based. It would be tempting to think that perhaps it is derived directly from the seaweed. However, this is very unlikely, as the colour of the seaweed comes from a mixture of pigments some of which are proteins which would be broken down by the digestive process in the copepod.
BTW, the flattened character of the copepod is considered to be an adaptation to living on the surface of the seaweed. Presumably it is less likely to be pulled off the plant by currents and waves.
